BETHESDA, Md. (WUSA9) -- Fifteen people were transported to a local hospital for carbon monoxide poisoning at a high rise apartment building, six are in serious condition, officials say.

A total of 150 residents of Middlebrooke apartments, located on the 5000 block of Battery Lane in Bethesda, were evacuated on Saturday at around 11 a.m., officials from the Montgomery County Fire Department said.

Resident Rob Marcarelli heard the alarm, he headed down eight flights of stairs when his friend fell to the ground.

"She fainted and hit her head on the stairwell. This is a pretty crazy scene, I've never really seen anything like it."

Out of the 15 residents who had to be taken to the hospital, two of them, were flown to a trauma center, including a maintenance worker.

Fire, EMS and Hazmat teams responded and treated the sick on sight.

Montgomery County Assistant Chief Scott Graham says there was a malfunction in the building's back-up generator.

"There were building maintenance workers downstairs. The generator was running. We don't know how long it was running or why. But it wasn't ventilating properly. He was overcome. One other person was overcome. Those two people were flown to a trauma center."

Their condition is serious but non-life threatening.

The generator had been running for some time because unacceptable levels were measured throughout the high rise building.

"We found levels from 200-800 parts per million. What's normal? We like it less than 35," Graham said.

With the fire station right across the street firefighters responded quickly and went through the complex floor by floor.

Several hours later residents made their way back into their apartments.

"Until I was coming down the stairs I could smell the gas. I feel very fortunate," one resident said.

Graham added, "This week starts fire prevention week. If there is a take away a CO detector and smoke alarm is a great thing."

Carbon Monoxide detectors are not required in the state of Maryland. Investigators are trying to determine if there were any CO detectors in the building.